Patent Application No. 2003/1031724A1, entitled “Cylinder With Optical Position Sensing Device And Method,” is directed to a fluid-pressure actuated piston/cylinder assembly having an optical sensor capable of reading indicia markings on a piston rod. The publication discloses various ways for providing optically readable scales upon a piston rod. Other coding scales are also disclosed, such as a bar code for determining the position of a movable member. Several methods for marking a rod with a scale are disclosed, including the use of a laser to create discolorations in a rod.
Sensing systems utilizing optical marks with an optical pickup present various problems. For example, the ability of an optical sensing head to read indicia depends upon the clarity of such indicia. In hostile environments, such as the environments typical for fluid actuated cylinders, the indicia marks are exposed to the environment, and consequently, are prone to degradation. Additionally, the piston rod requires specialized fabrication techniques for imparting the indicia on the rods, and a specialized sealing arrangement must be used for the optical sensing head.
Furthermore, an optical sensor solution relying upon quadrature measuring techniques has the same limitation as various inductive devices utilizing quadrature measuring techniques. Such inductive measuring devices rely upon a ferrous target having square ridges or teeth and a quadrature scheme for measuring relative position of the target with respect to a fixed sensing head. Resolution is therefore limited by the distances between the increments. In an optical system, resolution is limited by the distance between indicia marks. In many applications, higher resolution is required.